The present invention is concerned with a method of manufacturing elongate components. More particularly, the present invention is concerned with a method of manufacturing elongate aircraft components such as aircraft wing stringers by inducing plastic deformation by a differential contact three point bending operation.
Components such as aircraft stringers need to be shaped to the wing aerodynamic profile. In order to achieve this, stringers are plastically deformed by differential contact three point bending. By “differential contact three point bending” we mean a process by which the stringer is supported at two spaced positions on a first side. A punch is applied therebetween from a second, opposite side in order to induce a bending moment to cause local plastic deformation. Because the stringer deforms as the punch progresses, the contact area with the punch and the supports may change. This is not a classical three point bending load case (the forces are not point loads) and as such is defined as “differential contact”.
One problem with this method is that the stringer will spring back after the bending moment is released due to its elasticity. Known methods of forming are non-predictive. The operator will attempt to estimate the amount of punch movement required to provide a given plastic deformation. The punch is applied to the stringer by the estimated value and the plastic deformation is measured once the punch is retracted. If the level of deformation is too low, the operator will estimate a further punch distance and reapply the punch. Successive bending operations are applied at the same position until the required deformation is achieved. The punch information is then stored and applied to the next component, and so on until the required “first time” punch movement is refined to a satisfactory degree.
A typical 18 m stringer will be have up to 250 punching locations along its length. As such it is desirable to reduce the number of punching operations at each station.
Should the stringer be overdeformed (i.e. undergo too much deformation when punched), scrapping the part is not feasible as such parts are very expensive. As such, an inverse bending moment (again, estimated by the operator) is applied to the component to reverse the deformation. Such repeated and reverse application of plastic deformation to the stringer can cause problems such as work hardening and fatigue.
Because of the complex and changing geometry of the stringers used in the aerospace sector, prediction of the stringer plastic deformation by analytical methods is not appropriate.
An alternative is to use numerical simulation, such as finite element analysis (FEA) to predict the deformation of the stringer at each punch location. Not only would the analysis of a single stringer need to be repeated at each punch stage (in order to arrive at the required plastic deformation), but because of the changing cross section of the stringer along its length, these analyses would need to be carried out for each discrete punch position. This would be extremely time consuming and costly with respect to computing resource.